1. Field of the Invention
The present invention relates to a relay apparatus and a relay method.
2. Description of the Related Art
As techniques for improving throughput in radio communications, a MIMO (Multiple Input Multiple Output) scheme or a multi-antennas scheme is present. The MIMO scheme improves the throughput through simultaneous signal transmissions from multiple antennas at a uniform frequency.
FIG. 1 schematically illustrates the MIMO scheme. In the MIMO scheme, n streams, stream 1 to stream n, are transmitted from n antennas in a transmitter separately and are received at n antennas in a receiver via different radio propagation paths. The receiver can use the spacially different radio propagation paths to reconstruct the transmitted information piece 1 to piece n through signal detection of the n streams, stream 1 to stream n, from the n received signals.
In this manner, the MIMO scheme makes use of differences of the radio propagation paths among the multiple transmitter antennas and the multiple receiver antennas. Accordingly, in an exemplary case where various radio propagation paths are present due to existence of an obstacle along the radio propagation paths (multipath environment case), the receiver can easily achieve the signal detection in accordance with the MIMO scheme. However, the reception power tends to be attenuated due to scattering of radio waves caused by the obstacle, as illustrated in FIG. 2. On the other hand, in an exemplary line-of-sight environment, the radio propagation paths slightly differ from each other, which makes the signal detection at the receiver difficult. In the line-of-sight environment, however, the reception power does not drop substantially, as illustrated in FIG. 3.
For this reason, in the line-of-sight environment, it is difficult to improve the throughput in accordance with the MIMO scheme. In order to address this problem, it is conceived that a relay apparatus is provided between the transmitter and the receiver. The receiver receives a direct wave incoming from the transmitter directly and a relayed wave traveling from the transmitter via the relay apparatus and performs the signal detection on their composite wave. By using the relay apparatus to make the radio propagation paths intentionally complicated, the receiver could easily perform the signal detection even in the line-of-sight environment. Some conventional MIMO based relay methods are described in JP 2006-148482, for example.
However, even if the installation of the relay apparatus enables the receiver to receive the direct wave and the relayed wave, there are some cases where the signal detection by the receiver in accordance with the MIMO scheme remains difficult.
FIG. 4 illustrates one of the above cases. In the left side in FIG. 4, a transmitter, a relay apparatus and a receiver are schematically illustrated. The receiver receives a direct wave incoming from the transmitter directly and a relayed wave incoming via the relay apparatus and synthesizes the direct wave and the relayed wave. In general, the relay wave has a phase different from that of the direct wave. For convenience, it is assumed that the phase difference between the direct wave and the relayed wave is equal to φ. Then, the synthesis of the direct wave and the relayed wave can be achieved through vector addition. As a result, if the relayed wave has an extremely small amplitude, for example, the composite wave would be approximate to the direct wave, as illustrated in the upper-right part in FIG. 4. In the case where the composite wave is approximate to the direct wave, the receiver could substantially use only the direct wave for the signal detection, which may make the signal detection difficult as in the case of the line-of-sight environment. On the other hand, if the direct wave has an extremely small amplitude, the composite wave would be approximate to the relayed wave, as illustrated in the lower-right part in FIG. 4. In the case where the composite wave is approximate to the relayed wave, the receiver could substantially use only the relayed wave for the signal detection, which may make the signal detection difficult as in the case of the line-of-sight environment. In this manner, in accordance with the conventional scheme, even if the relay apparatus is utilized in the MIMO scheme, the signal detection may encounter the difficulties.